Generally one doesn't "revise" a work-for-hire agreement. You either agree to a work-for-hire arrangement or you don't. There's really no grey area.
By nature, any work-for-hire agreement will grant one party (the client) all rights to everything which is created. You don't "modify" that. You simply refuse to agree to it.
In most cases a work-for-hire agreement is presented because it provides HUGE benefits to the client. It somewhat sounds as if you are not being financially compensated for your work, or there's no existing compensation agreement in place for any work you've done. That's merely bad business.
A book credit will most likely never grant any real return. Your name may as well be written on a bathroom wall - the amount of referrals will generally be pretty much the same. Unless the item you get credit in is industry-specific to your industry, i.e. get credit in a book about logos, then you may get a great deal more referrals for logo design. That sort of thing. Typically credits are in addition to financial compensation not in lieu of it. Or a combination — discounted financial costs in exchange for a credit in the product.
I would guess that someone the client knows suggested a work-for-hire agreement so the client ultimately owns everything and would be free to reuse, repurpose or resell it all without giving you anything. Truth of the matter is, you need to know the motivations for presenting it. Especially if there's already a contract in place.
If there is an existing contract, any new contract will take precedence. If the new contract is a work-for-hire contract, whether or not that can be applied to past work would be something to ask an attorney. As well as specific rewording or concerns should also be presented to an attorney for review. A bunch of designers on some random web site are really not the people you want giving you legal advice.
The only time I ever consider a work-for-hire agreement is if I don't care about retaining ownership the work for future use, don't care what the client does with my work, and I'm being adequately compensated financially for the work. Matching those three factor aren't as common as one would think.
In any event, a portfolio is most often seen as "fair use" and you are typically free to display your projects in your portfolio regardless of a contract unless the contract specifically contains non-disclosure clauses.
